Traditional grasping uses normal forces to hold objects. If an object is small relative to the gripper, the gripper wraps around the object and applies normal forces to support it (form closure). For larger objects, the gripper squeezes the object and creates friction to the hold it (force closure).
Often, both direct support and friction hold the object. While friction is very useful for grasping objects, it has two drawbacks: the squeezing normal force can crush delicate or deformable objects, and the normal force tends to push objects out of the grasp in the case where the gripper cannot reach at least halfway around the object. Examples of such grippers are numerous, including rigid and fully actuated ones, as well as more compliant, under-actuated and back drivable grippers.
There exist a number of alternatives to traditional grippers. Vacuum is often used in manufacturing for lifting non-porous objects without readily graspable features. Vacuum can be combined with particle jamming and friction to grasp a variety of objects. A tacky mushroom-tipped adhesive can be used to lift objects once it is pressed onto the surface, and is able to lift over 0.4 N at≈2 kPa. These examples use primarily a normal force that is directed away from the object to lift it. Electrostatic adhesion requires much less pressing force to engage but provides limited adhesion that would require a very large piece and additional control infrastructures for large load applications.
In contrast to both traditional grippers as well as the above mentioned alternative grippers, the present invention advances the art by providing grippers based on shear adhesion, or adhesion that is tangent to a surface.